Functional diversity performs a key role in the isolation of nitrogen-fixing and phosphate-solubilizing bacteria from soil

Cuminaldehyde and Tobramycin Forestall the Biofilm Threats of Staphylococcus aureus: A Combinatorial Strategy to Evade the Biofilm Challenges

Staphylococcus aureus, an opportunistic Gram-positive pathogen, is known for causing various infections in humans, primarily by forming biofilms. The biofilm-induced antibiotic resistance has been considered a significant medical threat. Combinatorial therapy has been considered a reliable approach to combat antibiotic resistance by using multiple antimicrobial agents simultaneously, targeting bacteria through different mechanisms of action. To this end, we examined the effects of two molecules, cuminaldehyde (a natural compound) and tobramycin (an antibiotic), individually and in combination, against staphylococcal biofilm. Our experimental observations demonstrated that cuminaldehyde (20 μg/mL) in combination with tobramycin (0.05 μg/mL) exhibited efficient reduction in biofilm formation compared to their individual treatments (p < 0.01). Additionally, the combination showed an additive interaction (fractional inhibitory concentration value 0.66) against S. aureus. Further analysis revealed that the effective combination accelerated the buildup of reactive oxygen species (ROS) and increased the membrane permeability of the bacteria. Our findings also specified that the cuminaldehyde in combination with tobramycin efficiently reduced biofilm-associated pathogenicity factors of S. aureus, including fibrinogen clumping ability, hemolysis property, and staphyloxanthin production. The selected concentrations of tobramycin and cuminaldehyde demonstrated promising activity against the biofilm development of S. aureus on catheter models without exerting antimicrobial effects. In conclusion, the combination of tobramycin and cuminaldehyde presented a successful strategy for combating staphylococcal biofilm-related healthcare threats. This combinatorial approach holds the potential for controlling biofilm-associated infections caused by S. aureus.

Piperine, a Plant Alkaloid, Exhibits Efficient Disintegration of the Pre-existing Biofilm of Staphylococcus aureus: a Step Towards Effective Management of Biofilm Threats

Staphylococcus aureus causes a range of chronic infections in humans by exploiting its biofilm machinery and drug-tolerance property. Although several strategies have been proposed to eradicate biofilm-linked issues, here, we have explored whether piperine, a bioactive plant alkaloid, can disintegrate an already existing Staphylococcal biofilm. Towards this direction, the cells of S. aureus were allowed to develop biofilm first followed by treatment with the test concentrations (8 and 16 µg/mL) of piperine. In this connection, several assays such as total protein recovery assay, crystal violet assay, extracellular polymeric substances (EPS) measurement assay, fluorescein diacetate hydrolysis assay, and fluorescence microscopic image analysis confirmed the biofilm-disintegrating property of piperine against S. aureus. Piperine reduced the cellular auto-aggregation by decreasing the cell surface hydrophobicity. On further investigation, we observed that piperine could down regulate the dltA gene expression that might reduce the cell surface hydrophobicity of S. aureus. It was also observed that the piperine-induced accumulation of reactive oxygen species (ROS) could enhance biofilm disintegration by decreasing the cell surface hydrophobicity of the test organism. Together, all the observations suggested that piperine could be used as a potential molecule for the effective management of the pre-existing biofilm of S. aureus.

Rhizobium mayense sp. Nov., an efficient plant growth-promoting nitrogen-fixing bacteria isolated from rhizosphere soil

The nitrogen-fixing bacterium has great prospects in replacing synthetic fertilizers with biofertilizers for plant growth. It would be a useful tool in eradicating chemical fertilizers from use. Five nitrogen-fixing bacteria were isolated from the Tea and Groundnut rhizosphere soil out of which RSKVG 02 proved to be the best. The optimized condition of RSKVG 02 was found to be pH 7 at 30 °C utilizing 1% glucose and 0.05% ammonium sulfate as the sole carbon and nitrogen source. Plant growth-promoting traits such as IAA and ammonia were estimated to be 82.97 ± 0.01254a μg/ml and 80.49 ± 0.23699a mg/ml respectively. Additionally, their phosphate and potassium solubilization efficiency were evaluated to be 46.69 ± 0.00125 b mg/ml and 50.29 ± 0.000266 mg/ml. Morphological, and biochemical methods characterized the isolated bacterial culture, and molecularly identified by 16 S rRNA sequencing as Rhizobium mayense. The isolate was further tested for its effects on the growth of Finger millet (Eleusine coracana) and Green gram (Vigna radiata) under pot conditions. The pot study experiments indicated that the bacterial isolates used as bio inoculants increased the total plant growth compared to the control and their dry weight showed similar results. The chlorophyll content of Green gram and Finger millet was estimated to be 19.54 ± 0.2784a mg/L and 15.3 ± 0.0035 mg/L which suggested that Rhizobium sp. Possesses high nitrogenase activity. The enzyme activity proved to use this bacterium as a biofertilizer property to enhance soil fertility, efficient farming, and an alternative chemical fertilizer. Therefore, Rhizobium mayense can be potentially used as an efficient biofertilizer for crop production and increase yield and soil fertility.

Unraveling Mechanisms and Impact of Microbial Recruitment on Oilseed Rape (Brassica napus L.) and the Rhizosphere Mediated by Plant Growth-Promoting Rhizobacteria

Plant growth-promoting rhizobacteria (PGPR) are noticeably applied to enhance plant nutrient acquisition and improve plant growth and health. However, limited information is available on the compositional dynamics of rhizobacteria communities with PGPR inoculation. In this study, we investigated the effects of three PGPR strains, Stenotrophomonas rhizophila, Rhodobacter sphaeroides, and Bacillus amyloliquefaciens on the ecophysiological properties of Oilseed rape (Brassica napus L.), rhizosphere, and bulk soil; moreover, we assessed rhizobacterial community composition using high-throughput Illumina sequencing of 16S rRNA genes. Inoculation with S. rhizophila, R. sphaeroides, and B. amyloliquefaciens, significantly increased the plant total N (TN) (p < 0.01) content. R. sphaeroides and B. amyloliquefaciens selectively enhanced the growth of Pseudomonadacea and Flavobacteriaceae, whereas S. rhizophila could recruit diazotrophic rhizobacteria, members of Cyanobacteria and Actinobacteria, whose abundance was positively correlated with inoculation, and improved the transformation of organic nitrogen into inorganic nitrogen through the promotion of ammonification. Initial colonization by PGPR in the rhizosphere affected the rhizobacterial community composition throughout the plant life cycle. Network analysis indicated that PGPR had species-dependent effects on niche competition in the rhizosphere. These results provide a better understanding of PGPR-plant-rhizobacteria interactions, which is necessary to develop the application of PGPR.

Biochar and Rhizobacteria Amendments Improve Several Soil Properties and Bacterial Diversity

In the current context, there is a growing interest in reducing the use of chemical fertilizers and pesticides to promote ecological agriculture. The use of biochar and plant growth-promoting rhizobacteria (PGPR) is an environmentally friendly alternative that can improve soil conditions and increase ecosystem productivity. However, the effects of biochar and PGPR amendments on forest plantations are not well known. The aim of this study is to investigate the effects of biochar and PGPR applications on soil nutrients and bacterial community. To achieve this goal, we applied amendments of (i) biochar at 20 t hm⁻², (ii) PGPR at 5 × 10¹⁰ CFU mL⁻¹, and (iii) biochar at 20 t hm⁻² + PGPR at 5 × 10¹⁰ CFU mL⁻¹ in a eucalyptus seedling plantation in Guangxi, China. Three months after applying the amendments, we collected six soil samples from each treatment and from control plots. From each soil sample, we analyzed several physicochemical properties (pH, electrical conductivity, total N, inorganic N, NO₃⁻-N, NH₄⁺-N, total P, total K, and soil water content), and we determined the bacterial community composition by sequencing the ribosomal 16S rRNA. Results indicated that co-application of biochar and PGPR amendments significantly decreased concentrations of soil total P and NH₄⁺-N, whereas they increased NO₃-N, total K, and soil water content. Biochar and PGPR treatments increased the richness and diversity of soil bacteria and the relative abundance of specific bacterial taxa such as Actinobacteria, Gemmatimonadetes, and Cyanobacteria. In general, the microbial composition was similar in the two treatments with PGPR. We also found that soil physicochemical properties had no significant influence on the soil composition of bacterial phyla, but soil NH₄⁺-N was significantly related to the soil community composition of dominant bacterial genus. Thus, our findings suggest that biochar and PGPR amendments could be useful to maintain soil sustainability in eucalyptus plantations.

Exploration of strategies to increase the nitrogen and phosphate content of solid waste landfill soil

Several strategies were undertaken to increase the fertility of landfill soil as rapid urbanization remarkably decreases the agricultural land, posing challenges to the fast-growing human population. Towards this direction, soil microcosms were prepared wherein the addition of nutrient or biofertilizer or the combination of both increased the soil nitrogen and phosphate content considerably. The maximum amount of nitrogen fixation and phosphate solubilization occurred in microcosm treated with biofertilizer and nutrient. To investigate the underlying cause, we observed that separate application of nutrient or biofertilizer or combined application of both increased the abundance of nitrogen-fixing and phosphate-solubilizing bacteria in the microcosms. However, the highest abundance of nitrogen-fixing and phosphate-solubilizing bacteria was spotted in a microcosm challenged with nutrient and biofertilizer together. It was detected that with increasing population of nitrogen-fixing and phosphate-solubilizing bacteria, the soil nitrogen and phosphate level also got enhanced, respectively, thus establishing a strong positive correlation between them. The microcosm treated with biofertilizer and nutrient manifested the highest degree of heterotrophic microbial growth and microbial activity than the microcosms either treated with nutrient or biofertilizer. The microcosm treated with nutrient and biofertilizer was found to exhibit the highest functional diversity compared to others. A surface plot was constructed to demonstrate the association among microbial activity, functional diversity, and the availability of soil nitrogen and phosphate content of soil. The result indicates that the combined application of nutrient and biofertilizer increases the microbial activity leading to the formation of a heterogeneous ecosystem that enhances the nitrogen and phosphate content of landfill soil considerably.

Bioaugmentation of soil with Enterobacter cloacae AKS7 enhances soil nitrogen content and boosts soil microbial functional-diversity

Biofertilizer happens to be a promising alternative of chemical fertilizer in the establishment of sustainable agricultural practices. Following this observation, several nitrogen-fixing bacteria were isolated from the soil in which an isolate (AKS7) was selected for further studies as AKS7 showed considerable competence in growth on nitrogen-free growth medium. Acetylene reduction assay confirmed that AKS7 can fix atmospheric nitrogen efficiently. The result of Kjeldahl assay revealed that the organism (AKS7) could fix nitrogen up to 12 mg in 8 days. A strong positive correlation (r = 0.987) was observed between microbial cell biomass and the amount of nitrogen fixed by AKS7 over a period of 8 days. The organism was identified as Enterobacter cloacae through molecular and biochemical tests. To investigate the in situ nitrogen fixation by E. cloacae AKS7, naturally attenuated (AKS7 not-inoculated) and bioaugmented (AKS7-inoculated) soil microcosms were prepared. The bioaugmented microcosm showed higher level of soil nitrogen content than naturally attenuated microcosm. A large number of heterotrophic as well as nitrogen-fixing microorganisms were counted in bioaugmented microcosm than naturally attenuated microcosm. Results of the carbon source utilization patterns of BiOLOG ECO plate revealed that bioaugmented microcosm exhibited higher level of functional richness and evenness that lead to the exhibition of higher level of microbial functional-diversity in bioaugmented microcosm than the naturally attenuated microcosm. Taken together, the results indicated that augmentation of E. cloacae AKS7 into soil enhanced the nitrogen content and soil microbial functional-diversity considerably.